Molecular characterization of a 40-kDa outer membrane protein, FomA, of Fusobacterium periodonticum and comparison with Fusobacterium nucleatum.

The 40 kDa-outer membrane protein FomA of Fusobacterium periodonticum ATCC 33693 was found to exhibit heat modifiable properties, typical for a porin, and N-terminal sequencing indicated a close relationship to the porin FomA of Fusobacterium nucleatum. A polymerase chain reaction approach was therefore applied for sequencing the fomA gene of F. periodonticum, and nucleotide and deduced amino acid sequences were aligned and compared with the corresponding sequences of different strains of F. nucleatum. In all strains we found a common protein upstream of the fomA gene. The noncoding area upstream of the putative -35 region of the F. periodonticum fomA gene exhibited little sequence similarity with the F. nucleatum gene. The transcriptional unit of FomA, on the other hand, was very similar, with the similarities concentrated in domains that were interspersed with hypervariable regions. A topology model was made and compared with those made for F. nucleatum. This indicated that the great similarities reside in the membrane-spanning segments of the protein, while most cell surface exposed loops were hypervariable. The results strongly support the proposed model for FomA and also indicate that these taxa are related but on a lower level than the subspecies level. The codon usage of F. periodonticum is comparable to that of F. nucleatum, and the triplet AGA is the only codon used for arginine.